Gulf War veterans suffering mystery illnesses after returning from the 1991 conflict were probably not poisoned by pesticides, researchers have said.

Exposure to chemicals has been blamed for veterans' symptoms such as depression, poor sleep and mood swings.

But Bristol University researchers, writing in the Journal of Epidemiology and Community Health, blame other environmental toxins in this country.

It is thought more than 8,500 Gulf War troops have reported health problems.

We are getting 50 people contacting us every month with new claims

Sean Newton, National Gulf War Veterans and Families Association

Heavy exposure to organophosphate pesticides routinely sprayed on tents and other equipment is just one theory which has been examined by those investigating claims of unexplained illness among veterans.

Others include multiple vaccinations given to troops before deployment and exposure to nerve agents or depleted uranium during the war.

Several previous studies have found a link between jobs which involve regular exposure to organophosphates, such as sheep farming, and illness, but none has proven that the chemicals are the cause.

However, people who carry a version of a gene called PON1 appear less able to break down and dispose of organophosphates in their bodies, offering a potential clue as to why these chemicals might affect some people but not others.

Older women

The Bristol researchers did not examine Gulf War veterans directly, looking instead at a group of older women who were unlikely to have had any contact with organophosphates during their working lives.

They wanted to test whether those who carried the variant PON1 gene were just as vulnerable to the same symptoms as the veterans, without the involvement of organophosphates.

They found that this was true - the women with the variant were significantly more likely to report symptoms of depression than those with other versions of PON1.

However, the absence of organophosphates from their lives meant that something else was likely to be the culprit in these cases.

The researchers said this meant that, although they could not rule out organophosphate poisoning in Gulf War veterans, a different as-yet-unknown toxin at home in the UK was more likely to be responsible.

Sean Newton, vice-chairman of the National Gulf War Veterans and Families Association, which has campaigned for recognition of so-called Gulf War Syndrome, called for further, independent research.

He said: "This problem is not going away - we are getting 50 people contacting us every month with new claims."

Professor Simon Wessely, from the Institute of Psychiatry in London, has helped carry out studies into the alleged links between the veterans' symptoms and organophosphates.

He said: "There's no doubt that in someone is exposed to the wrong dose, in the wrong place, these can be dangerous chemicals, but there were good reasons for using them in the Gulf.

"What we do know is that organophosphate poisoning is associated with peripheral nerve damage. Our study, and another, larger study in the US looked for signs of this in large number of veterans, and found no evidence of this kind of damage."